Forced limb-use effects on the behavioral and neurochemical effects of 6-hydroxydopamine.

Rats with unilateral depletion of striatal dopamine (DA) show marked preferential use of the ipsilateral forelimb. Previous studies have shown that implementation of motor therapy after stroke improves functional outcome (Taub et al., 1999). Thus, we have examined the impact of forced use of the impaired forelimb during or soon after unilateral exposure to the DA neurotoxin 6-hydroxydopamine (6-OHDA). In one group of animals, the nonimpaired forelimb was immobilized using a cast, which forced exclusive use of the impaired limb for the first 7 d after infusion. The animals that received a cast displayed no detectable impairment or asymmetry of limb use, could use the contralateral (impaired) forelimb independently for vertical and lateral weight shifting, and showed no contralateral turning to apomorphine. The behavioral effects were maintained throughout the 60 d of observation. In addition to the behavioral sparing, these animals showed remarkable sparing of striatal DA, its metabolites, and the expression of the vesicular monoamine transporter, suggesting a decrease in the extent of DA neuron degeneration. Behavioral and neurochemical sparing appeared to be complete when the 7 d period of immobilization was initiated immediately after 6-OHDA infusion, only partial sparing was evident when immobilization was initiated 3 d postoperatively, and no sparing was detected when immobilization was initiated 7 d after 6-OHDA treatment. These results suggest that physical therapy may be beneficial in Parkinson's disease.

Differential activities of E2F family members: unique functions in regulating transcription.

Several regulators of E2F transcriptional activity, including the retinoblastoma tumor suppressor (Rb) protein, p16Ink4a, cyclin D1, and cyclin-dependent kinase 4, have been shown to be targets for genetic alterations that underlie the development of human cancers. Deregulation of E2F transcription factors as a result of these genetic alterations is believed to contribute to tumor development. This hypothesis is supported by the finding that at least some members of the E2F gene family can contribute to oncogenic transformation when overexpressed. Each E2F family member can dimerize with DP proteins, bind consensus E2F sites, and activate transcription. Several pieces of evidence suggest, however, that the various E2F species have unique functions in regulating transcription. We compared the abilities of E2F1, E2F4, and E2F5 to activate transcription from a variety of gene promoters and found that in all cases E2F1 was the most potent activator, followed by E2F4 and then by E2F5. Construction of chimeric proteins between E2F1 and E2F4 demonstrated that either the carboxy terminus or the amino terminus of E2F1 could make E2F4 a more potent activator. In contrast, neither the carboxy terminus nor the amino terminus of E2F1 could significantly increase the activity of E2F5. We found that, consistent with a role for E2F5 in transcriptional repression, E2F5's binding partner p130, like Rb, could also actively repress transcription when directly bound to a target promoter.